Usually, a metal L-ascorbate such as sodium L-ascorbate is produced industrially as an intermediate product in the improved process for producing L-ascorbic acid, namely so-called Reichstein process [T. Reichstein and A. Grussner, Helv. Chim. Acta, 17, 311 (1943)] and is obtained from a lower alkyl 2-keto-L-gulonate such as methyl 2-keto-L-gulonate. In general, according to this process, the lower alkyl ester is obtained by esterification of diacetone-2-keto-L-gulonic acid (DAGA.H.sub.2 O) with a lower alcohol in the presence of sulfuric acid, and the yield of the metal L-ascorbate is limited due to equilibrium of the esterification. Then, in Reichstein process, usually heating is carried out for a relatively long period of time to increase the degree of esterification and thus attain the satisfactory degree of esterification. However, such a heating adversely influences on purity.
In order to increase purity, for example, a process wherein sodium bicarbonate is added to a reaction mixture after esterification to precipitate impurities, followed by filtration (JP-A 3-38579) has been known.
However, such a known process is accompanied by difficulties in the operation from the viewpoint of industrial production. Further, it is necessary to treat the filtrate separated again to recover the esterified product therein and it is not always an advantageous process from the economical viewpoint.